One property of a PN diode is that it conducts an electric current in one direction and blocks it in the other. This behavior arises from the electrical characteristics of a junction, called a PN junction fabricated within a semiconductor crystal. When P-type and N-type materials are placed in contact with each other, the junction behaves differently than either type of material alone. Current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode. This non-reversing behavior arises from the nature of the charge transport process in the two types of materials. PN diodes are used in a number of microelectronic devices including, but not limited to: transistors, field effect transistors, and memory devices.